users: reword ban notice messages.

* Show the ban length instead of the ban expiration date in ban notices.
* Fix the ban notice to not say "Your account has been temporarily
  banned" when it's a permanent ban.

app/views/bans/show.html.erb

@@ -6,7 +6,7 @@
     <h1>Show Ban</h1>
     <ul style="margin-bottom: 1em;">
       <li><strong>User</strong> <%= link_to_user(@ban.user) %></li>
-      <li><strong>Duration</strong> <%= humanized_duration(@ban.created_at, @ban.expires_at) %></li>
+      <li><strong>Duration</strong> <%= humanized_duration(@ban.duration) %></li>
       <li>
         <strong>Reason</strong>
         <div class="prose">